A light detector may utilize a photomultiplier tube (PMT) to detect light, particularly from weak or dim sources of light. The PMT is a vacuum tube that encloses a photocathode, an electron multiplier, and an anode. In accordance with the photoelectric effect, incident photons striking the photocathode cause the photocathode to emit electrons (photoemission). The electrons are multiplied by the electron multiplier, which comprises a series of dynodes. The dynodes cause successive instances of secondary electron emission, ultimately producing enough electrons to generate a useful current that can be correlated to photon counts or intensity. For many optical-based measurement applications, for example fluorescence and luminescence measurement, the PMT may be considered to be the preferred type of light detector component in view of its relatively low cost, high gain, high frequency response, large numerical aperture, and capability for single photon counting.
Optical-based analytical instruments that often utilize PMTs include optical plate readers commonly employed in the life science industry (e.g., for biochemistry, cell biology, immunology, molecular biology, and microbiology). Such instruments typically take measurements from samples in microplates. Such instruments may be configured to perform a specific type of measurement (e.g., fluorescence, luminescence, absorbance, cell imaging, etc.), or may be capable of performing multiple types of measurements at the selection of the user. The latter type of instrument is often termed a “multimode” analytical instrument or multimode reader. Certain multimode readers are configured to receive application-specific cartridges that enable a user to select the type of experiment to be performed on a sample. The selected cartridge is coupled to the instrument whereby the instrument is properly configured for carrying out the selected experiment. The cartridge may contain optics specific to or optimized for a particular type of application. The internal optics housed within the cartridge may communicate with external optics housed within the instrument through optical ports of the cartridge's housing. Some cartridges may additionally include an internal light source and/or light detector. Some examples of cartridge-based multimode readers are described in, for example, U.S. Patent Application Pub. No. 2014/0191138 and U.S. Pat. No. 8,119,066, the entire contents of which are incorporated by reference herein in their entireties.
An optical-based analytical instrument may utilize two different types of PMT to optimize for both fluorescence and luminescence applications. One example is when the fluorescence detection wavelength range needs to go higher than 700 nanometers (nm). In this case a red-sensitive PMT is needed to support the fluorescence wavelength range, and a visible wavelength-only (VIS-only) PMT is needed for luminescence measurement to avoid the higher dark counts or dark current of red-sensitive PMTs. Measuring luminescence with a red-sensitive PMT can limit performance significantly. However, for reasons of cost and/or other reasons, it would be desirable to provide an optical-based analytical instrument that includes a single PMT capable of performing well in different types of experiments. A solution to this problem is to adequately cool a PMT to reduce dark counts or dark current, thereby avoiding the need for a second PMT. Cooled PMTs are commercially available, but are unacceptably large or bulky for integration into a typical optical-based analytical instrument in which space is already limited and for which overall size or footprint is a concern of the user. Moreover, known cooled PMTs often allow condensation to develop on the PMT, which may contaminate the sample under investigation or the sensitive optics and/or electronics of the instrument. Generally, condensation will occur on any cooled surface exposed to moisture-containing air.
Therefore, there is a need for a cooled PMT or PMT-based light detector that is compact and reduces or prevents condensation, and an apparatus or system that includes such a PMT or PMT-based light detector.